1. Field of the Invention
The present invention is directed to a method of recovering silicon carbide whiskers from a mixture of silicon carbide whiskers and carbonaceous silicon carbide particles. More particularly, the present invention is directed to such recovery by using a series of specified steps, including selective flocculation-liquid/liquid extraction techniques.
2. Prior Art
Silicon carbide whiskers have in recent years been found to be particularly useful for reinforcing metal and other composite materials so as to enhance the physical properties thereof. Thus, for example, U.S. Pat. No. 4,060,412 to A. P. Divecha, describes the use of microscopic silicon carbide fibers in a mixture with metal particles so as to be randomly oriented, and the subsequent extrusion and casting to form very high strength composite material billets.
Silicon carbide whiskers may be produced using elemental materials such as is described in U.S. Pat. No. 3,758,672, wherein solid phase carbon and solid phase silicon are heated to specified temperatures in a hydrogen and chlorine atmosphere to form silicon carbide whiskers on a substrate. Alternatively, silicon carbide whiskers have been produced from natural materials such as is described in U.S. Pat. No. 3,754,076, wherein the silicon carbide whiskers are formed from the silica and the carbon which are naturally present in rice hulls. Each technique has its advantages and disadvantages. However, the method of producing silicon carbide from rice hulls appears to be more advantageous in that it is a method whereby one of the most abundant waste materials in the world is consumed. Nonetheless, when rice hulls are used as a starting material for the production of silicon carbide whiskers, a substantial amount of non-whisker residual material remains and an area of developing interest involves the separation of the silicon carbide whiskers from the other residual materials. To date, it is believed that relatively well-known mechanical screening and sifting means have been employed to separate the silicon carbide whiskers from residual materials with a minimum success, when formed from rice hulls.
Although the prior art is replete with many techniques for separating various solid materials from one another, it is believed that the method of the present invention, employing a selective flocculation-liquid/liquid extraction as well as other necessary steps, has not heretofore been contemplated or rendered obvious. Exemplary of the prior art is U.S. Pat. No. 3,243,284, which describes a method for collecting metal whiskers. This patent describes a process wherein the metal whisker growth substrate is immersed in mercury and wherein the mercury and substrate are vibrated and a liquid is added to the mercury which is non-reactive with respect to the whiskers and which is mixed with the mercury to transport the whiskers from the mercury into the liquid, followed by filtering. U.S. Pat. No. 3,439,801 is directed to a process for removing discoloring impurities which may be organic or graphitic in nature from a clay slip. The method described in this patent involves intimately contacting an aqueous slip of fine milled clay with a water insoluble, non-ionizable organic liquid and thereupon recovering the clay from the separated clay water base. The organic liquid is the sole chemical reagent necessary for the effective separation of the discoloring impurities from the fine milled clay in the slip. While this patent is directed to a liquid/liquid separation technique involving the use of water and a water insoluble non-ionizable organic liquid, it is not directed to the separation of whisker materials from other solids, much less the separation of silicon carbide whiskers made from rice hulls.
U.S. Pat. No. 3,462,262 describes a process for the recovery of excess carbon from the product of an iron ore direct reduction process. The method involves treating the iron ore containing the carbon with oil followed by flotation in water. U.S. Pat. No. 3,764,007 is directed to the separation of solid particles from solid particles having a different shape. The invention therein is directed to a process for such a separation using a suspension, without turbulence, at a specified velocity profile using rotary movement of two discs. U.S. Pat. No. 3,865,315 is directed to a process for separating fibrous material such as wool fiber from mixtures of fibrous material and adhered shot without substantially destroying the fiber length. This method involves passing a water suspension of the fibrous material and adhered shot through an attrition device to gently abrade the fiber shot interface, then diluting the abraded mixture with water, agitating the diluted mixture, and decanting to separate the substantially unbroken fibers from the now separated shot. While this patent is directed to removing fibrous material from shot material, it does not employ the present invention separation technique but rather relies ultimately upon gravity for removal of the shot.
U.S. Pat. No. 3,920,446 is directed to a method for treating siliceous materials to form silicon carbide for use in refining ferrous material. The method involves heating mixtures of SiC and iron product at elevated temperatures comminuting the resulting product from the heat treatment and separating the metallic iron from the silicon carbide magnetically. U.S. Pat. No. 3,836,356 is also directed to a process wherein metal iron is removed from silicon carbide residue magnetically. U.S. Pat. No. 3,665,066 is directed to beneficiation of coals using liquid agglomeration techniques and U.S. Pat. No. 3,268,071 is likewise directed to a process for the separation of solids by agglomeration using liquid/liquid techniques. U.S. Pat. No. 4,118,464 is directed to a process for separating silica from silicate bearing materials and includes discussion of the formation and separation of silicon carbide from such materials. It is disclosed that the silicon carbide fibers obtained by the process may be separated by a suitable conventional mechanical screen process at which time there may be obtained an excellent separation of the silicon carbide from a carbon matrix residue and other materials. While this patent is directed to a separation of silicon carbide from carbonaceous materials, it does not describe the selective flocculation-liquid/liquid separation technique of the present invention.
O. Mellgren et al, Trans, Inst. Min. Metall., Vol. 75, pp. C267-C268, 1966 describe a method for recovering ultrafine mineral particles by extraction with an organic phase and an aqueous phase. R. W. M. Lai et al, Transactions of Society of Mining Engineers, AIME, Vol. 241, December, 1968, pp. 549-556, describe liquid/liquid extraction of ultrafine particles using water-oil separation, and J. R. Farnand et al, The Canadian Journal of Chemical Engineering, April, 1961, pp. 94-97, describe spherical agglomeration of solids in liquid suspension. I. E. Puddington et al, in Minerals Science Engineering, Vol. 7, No. 3, October, 1975, describe spherical agglomeration processes and include a discussion on mineral beneficiation using liquid techniques.
Lastly recently filed co-pending application, Ser. No. 53,738, filed on July 2, 1979, entitled "Recovery of Silicon Carbide Whiskers from Coked, Converted Rice Hulls by Liquid-Liquid Separation" to B. K. Parekh et al, assigned to Silag, Inc., is directed to silicon carbide whisker recovery from a mixture of silicon carbide whiskers and carbonaceous silicon carbide particles. The invention involves shredding the mixture down to a specified particle size, dispersing the mixture in water to form an aqueous mixture, agitating the aqueous mixture, mixing the aqueous mixture with an immiscible organic liquid which is lighter than water, agitating the resulting water-organic liquid mixture, allowing the organic liquid and its contents to rise above the water and its contents, separating the two liquid phases into an organic liquid phase and an aqueous phase, and, lastly, performing a solid-liquid separation on each of the two phases, thereby obtaining from the aqueous solution the desired silicon carbide whiskers and obtaining from the organic liquid a carbonaceous silicon carbide particle product. Also, recently filed co-pending application Ser. No. 53,737, filed on July 2, 1979, entitled "Recovery of Silicon Carbide Whiskers from Coked, Converted Rice Hulls by Froth Flotation", to B. K. Parekh et al, assigned to Silag, Inc., is directed to silicon carbide whisker recovery from a mixture of silicon carbide whiskers and carbonaceous silicon carbide particles. The invention involves shredding the mixture down to a specified particle size, dispersing the mixture in water to form an aqueous mixture, agitating the aqueous mixture, adding surface active reagents to the aqueous mixture, agitating the resulting water-reagent mixture, subjecting the water-reagent mixture to froth flotation having at least three stages, removing tailings therefrom containing water and silicon carbide whiskers and removing float therefrom containing reagents and carbonaceous silicon carbide particles, performing a solid-liquid separation on the tailings and the float, thereby obtaining from the aqueous solution the desired silicon carbide whiskers and obtaining from the reagent solution a carbonaceous silicon carbide particle product.
In summary, of all the prior art described herein, there is no teaching of the present invention nor is there any teaching that would render the present invention obvious.